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Additively manufactured (and in particular laser powder bed fused) materials represent a manyfold challenge for the materials scientist and engineer because of their distinctive microstructure. If laser powder bed fusion is used to produce components, the complexity level increases because meso-structures (e.g., overhanging features, surface and internal defects) gain importance. Furthermore, if the main advantage of additive manufacturing, i.e., the freedom of design, is to be fully exploited, and geometrically complex structures, such as lattices, are manufactured, then such structures become meta-materials. This means that the geometry and the materials properties become equally important.
This matryoshka-like (more literary than the dry “multi-scale”) complexity makes the characterization of the residual stress fields by means of diffraction methods so difficult with the current means, that new paradigms are necessary to tackle the challenge.
Indeed, classic open problems acquire an extra layer of difficulty, such that new solutions need to be found and the sometimes-dormant debate needs to be re-opened. Examples include the determination of: a- the unstrained reference: this reference can become location-dependent and needs to be carefully determined; b- the so-called diffraction elastic constants, which becomes immensely challenging since even the single-crystal elastic constants are not known for additively manufactured materials.
On top of this, other problems arise. The determination of the principal axes of stress becomes non-trivial because the hatching strategy sometimes dominates over the sample geometry. Even further, in complex structures, such as lattices, the textbook statement that the strain measurement in six independent directions uniquely identifies the strain tensor becomes simply invalid. The peculiar surface features of additively manufactured materials transform trivial tasks into formidable challenges: the precise alignment of a specimen in a beam or the determination of surface stresses with laboratory X-rays need to be thoroughly re-discussed and lay far from being routine tasks.
In this paper, we will show a few examples of the cases mentioned above. We will demonstrate that sometimes the classic approach works very well, but other times surprising conclusions can be drawn from in-depth studies of the residual stress in additively manufactured materials. In short, we predicate that classic methods cannot be used on additively manufactured materials and structures without a critical evaluation of their validity and application range.
The evolution of the crystal structure and crystallographic texture of porous synthetic cordierite was studied by in situ high-temperature neutron diffraction up to 1373 K, providing the first in situ high-temperature texture measurement of this technologically important material. It was observed that the Crystal texture slightly weakens with increasing temperature, concurrently with subtle changes in the crystal structure. These changes are in agreement with previous work, leading the authors to the conclusion that high-temperature Neutron diffraction allows reliable crystallographic characterization of materials with moderate texture. It was also observed that structural changes occur at about the glass transition temperature of the cordierite glass (between 973 and 1073 K).
Crystal structure refinements were conducted with and without quantitative texture analysis being part of the Rietveld refinement, and a critical comparison of the results is presented, contributing to the sparse body of literature on combined texture and crystal structure refinements.
The combination of tomographic, microstructural data with other experimental techniques and with modeling is paramount, if we want to extract the maximum amount of information on material and component properties. In particular, quantitative image analysis, statistical approaches, direct discretization of tomographic reconstructions represent concrete possibilities to extend the power of the tomographic 3D representation to insights into the material and component performance. This logic thread equally holds for industrial and academic research, and valorizes expensive experiments such as those carried out at synchrotron sources, which cannot be daily repeated.
I will show a few examples of possible use of X-ray tomographic data for quantitative assessment of damage evolution and microstructural properties, as well as for non-destructive testing. Examples of micro-structured inhomogeneous materials will be given, such as Composites, Ceramics, Concrete, and Additively manufactured parts. I will also show how X-ray refraction computed tomography (CT) can be highly complementary to classic absorption CT, being sensitive to internal interfaces.
Additionally, I will present a new technique in our portfolio, Neutron Diffraction, which is extremely well suited to the study of internal stresses, both residual and under external load.
Combination of high-resolution time-of-flight (TOF) Neutron diffraction, scanning electron microscopy (SEM) and small angle neutron scattering (SANS) have been used to study hardening precipitation in 2014Al and 2124Al alloys. Neutron diffraction experiment revealed presence of tetragonal CuAl2 Phase (θ/θ´ precipitates) in 2014Al alloy and orthorhombic CuMgAl2 Phase (S-precipitates) in 2124Al alloy. SEM image analysis revealed that the θ/θ´-precipitates have platelet morphology while the S-precipitates exhibit rod-like shapes. From SANS data analysis the characteristic dimensions of precipitate particles and their polydisperse distributions were estimated.
Neutron diffraction: the forgotten non-destructive technique for residual stress analysis … and more
(2016)
3-D Stress Analysis
(Bulk) Stress mapping
Thick (and thin) films & Interfaces
Bulk high temperature
Real time
In-situ testing: Large sample environment (Stress rigs, Furnaces, …)
Neutrons and Synchrotron Radiation allow all this because they are FASTER , DEEPER and MORE PRECISE than lab equipment
(Flux)(Energy)(Parallel Beam)
Internal damage of an AlSi12CuMgNi alloy reinforced with planar randomshort fibres has been investigated after compression. This damage strongly influences the load partition between matrix and reinforcement. For fibres perpendicular to the applied load, breakage and interconnected cracks appear in significantly higher volume fraction than with fibres parallel to load.
We show that the equation proposed by Takeuchi and Argon to explain the creep behavior of Al–Mg solid solution can be used to describe also the creep behavior of pure aluminum. In this frame, it is possible to avoid the use of the classic pre-exponential fitting parameter in the power law equation to predict the minimum creep strain rate. The effect of the fractal arrangement of dislocations, developed at the mesoscale, must be considered to fully explain the experimental data. These ideas allow improving the recently introduced SSTC model, fully describing the primary and secondary creep regimes of aluminum alloys without the need for fitting. Creep data from commercially pure Al99.8% and Al–Mg alloys tested at different temperatures and stresses are used to validate the proposed ideas.
The evolution of the microstructure of multiphase metal matrix composite AlSi12CuNiMg with 7% vol. Al2O3 + 15% vol. SiC (short fibers and whiskers, respectively) was studied by synchrotron computed tomography. It comprehended as cast and after heat treatment conditions, as well as damage evaluation after ex-situ compression tests at room temperature. The volume fraction of different phases, their distribution, their orientation, and damage events are studied.
The influence on mechanical properties of the orientation of the planar random short fibres Al2O3 towards loading was investigated. Phase-specific load partition analysis for samples with fiber plane parallel and orthogonal to load, respectively, was performed by means of neutron diffraction (ND) during in-situ compression tests at room temperature. ND results proved that damage occurrence in the fillers strongly depends on the preferential orientation of those, playing a crucial role in the failure of the sample. The computed tomography observations confirm the damage observations from curves of load partition analysis of each phase.
Stress-induced damage evolution in cast AlSi12CuMgNi alloy with one- and two ceramic reinforcements
(2017)
Two composites, consisting of an as-cast AlSi12CuMgNi alloy reinforced with 15%vol. Al2O3 short fibres and with 7%vol. Al2O3 short fibres + 15%vol. SiC particles were studied. Synchrotron computed tomography disclosed distribution, orientation, and volume fraction of the different phases. In-situ compression tests during neutron diffraction in direction parallel to the fibres plane revealed the load partition between phases. Internal damage (fragmentation) of the Si phase and Al2O3 fibres was directly observed in CT reconstructions. Significant debonding between Al-matrix and SiC particles was also found. Finally, based on the Maxwell scheme, a micro-mechanical model was utilized for the new composite with two ceramic reinforcements; it rationalizes the experimental data, and predicts the evolution of all internal stress components in each phase.
Load partitioning between phases in a cast AlSi12CuMgNi alloy was investigated by in-situ compression test during neutron diffraction experiments. Computed tomography (CT) was used to determine volume fractions of eutectic Si and intermetallic (IM) phases, and to assess internal damage after ex-situ compression tests. The CT reconstructed volumes showed the interconnectivity of IM phases, which build a 3D network together with eutectic Si. Large stresses were found in IMs, revealing their significant role as a reinforcement for the alloy. An existing micromechanical model based on Maxwell scheme was extended to the present case, assuming the alloy as a three-phase composite (Al matrix, eutectic Si, IM phases). The model agrees well with the experimental data.
Moreover, it allows predicting the principal stresses in each phase, while experiments can only determine stress differences between the axial and radial sample directions. Finally, we showed that the addition of alloying elements not only allowed developing a 3D interconnected network, but also improved the strength of the Al matrix, and the ability of the alloy constituents to bear mechanical load.